1. Field of the Invention
The present invention relates generally to ink-jet technology, more particularly to ink-jet print modes, and more specifically to varying ink dot placement to minimize cyclic print errors.
2. Description of Related Art
The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol . 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No.1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy sic! Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
Generally, ink-jet printing involves movement and position tracking of ink-jet pens scanned (X-axis) across a print medium while the print medium is stepped transversely (Y-axis) in order that ink drops can be fired onto the print medium (Z-axis). Row and column dot matrix manipulation is used to turn the drops of ink into alphanumeric characters or graphic image patterns. Pen tracking, both movement and position, is usually controlled by employing magnetic or optical transducers and encoders, such as a strip encoder scale cooperating with an encoder or detector transducing or reading scale divisions. An example of an ink-jet apparatus encoder system is disclosed in U.S. Pat. No. 4,789,874, by Majette et al. (assigned to the common assignee of the present invention) for a Single Channel Encoder System, incorporated herein by reference.
In ink-jet printing, both dot density--with the current state-of-the-art being true 720 dot-per-inch ("dpi")--and ink drop placement have improved such that near-photographic quality graphic prints are now a commercial reality. With the use of special papers, the difference between a photograph and an ink-jet print made from a digitized scan of the photograph is hard to discern. As ink drop volume decreases and dot density rises, dot placement accuracy must improve and errors are exacerbated. For example, in double-dot-always print modes where one drop of ink is supposed to land precisely on top of a previous dot, when the drop volume is, for example, 32 picoliters ("pl"), a slight offset of the second drop should still provide for overlap and a small printing defect. But, an 8 pl drop misalignment at the same dpi may miss the target picture element ("pixel") and will produce a very noticeable print artifact. Smaller volume drops may actually land side-by-side rather than dot-on-dot, or vice-versa. Multi-level color printing, requiring the precise mixing of cyan, magenta, and yellow drops being fired from different primitives of a print head nozzle plate have the same problem. Random print errors have been virtually eliminated by half-toning techniques, such as error diffusion and dithering, and by using a variety of print modes, such as dot-on-dot print modes, double-dot-always print modes, dot-shingling print modes, bi-directional, superpixel, checkerboard print modes, and a variety of other methodologies known in the art. The types of remaining, noticeable, print errors--those visible to the naked eye upon close inspection of a print--are generally attributable to cyclic, systematic errors.
Cyclic errors are caused by hardware tolerance limitations, printer vibrations, drive gear and belt tooth ripple effects, and the like, that cause print errors to line up and become visible, diminishing the quality of a print. For example, ink-jet pens ride in carriages mounted on a slider bar and are driven by belt drives to scan across a sheet of paper at high speed, firing the minuscule droplets of ink on the fly from a plurality of nozzles. Dot placement on the paper is affected by mechanical tolerances for the pen shapes, pen mounts, pen and carriage datums, carriage mount to the slider bar, belt to carriage couplings, drive motor commutations, paper transport mechanisms--both electrical and mechanical--mechanical vibration harmonics caused by the relative motions, and electrical power fluctuations, or ripples, in both the system power supply for the print head and for the drive motor and the paper feed motor. Dot placement is thus a function of both paper axis directionality deviations and scan axis directionality deviations.
The use of current random error correction techniques allows cyclic errors to pile up on top of each other and become even more apparent artifact patterns in a print. In other words, one tolerance being slightly off can cause print errors and those errors will be cyclic, lining up in the print and effecting its quality. This is demonstrated by FIG. 4A. In FIG. 4A, dot size is magnified several hundred times and a single line feed error is simulated at 0.5 dot row. Note particularly that the white spaces between dots line up to form distinct patterns that are highly visible.
In U.S. Pat. No. 5,426,457 (assigned to the common assignee of the present invention), Raskin discloses a Direction-Independent Encoder Reading; Position Leading and Delay, and Uncertainty to Improve Bidirectional Printing. In a bidirectional print mode, Raskin sets up an asymmetrical dot-on-dot, drop firing timing scheme such that drops lead or approach the target picture element ("pixel") from opposite directions during successive passes in order to improve dot position accuracy. In order to solve a mottling problem (too much ink in one location, a particularly significant problem when printing on transparencies where ink absorption is relatively low and dry time is relatively high), for unidirectional printing Raskin introduces a deliberate noise to back off of the accuracy created by the asymmetrical timing scheme. Col. 21: 11. 19-col. 23: 11. 35. However, in the overall methodology, cyclic errors can still be a problem.
Therefore, there is a need for methods and apparatus to print high density ink-jet dot matrix data where compensation is provided to minimize cyclic error patterning.